Warren in Lancaster, PA
A side snipesbill is similar to a side rabbet in that they have a cutter on side of the plane that cuts on a vertical wall. A side rabbet can be used to widen a groove or clean up the side of a groove. However a quirk (a V shaped groove with one side a vertical wall and the other rounded) is too narrow for a side rabbet plane. It can be worked with a side snipesbill since its point allows it to get into the quirk.
These planes were on my list to buy forty years ago, but though I have made thousands of mouldings, I have not felt the need for them. The Seaton chest inventory (1796) has about seventy planes, including a pair of snipebills and a pair of side rabbets, but no side snipebills.
Rabbet planes, skew rabbets, side rabbets, sinking snipesbills, and side snipesbills are discussed by Nicholson (1812).